Electrically heatable glow plug and method for producing said electrically heatable glow plug

ABSTRACT

An electrically heatable glow plug and a method for manufacturing an electrically heatable glow plug are proposed that enable a protection of a heating coil of the glow plug against nitridation and evaporation of the aluminum from the heating conductor alloy. The glow plug includes a glow tube that is closed at the end, into which the electrically conductive heating coil is inserted, the heating coil being formed at least partially of aluminum, in particular of an aluminun-iron-chromium alloy. In the glow tube, oxygen donors are provided in order to form an aluminum oxide layer on the surface of the heating coil before or during the heating of the heating coil.

FIELD OF THE INVENTION

The present invention relates to an electrically heatable glow plug anda method for manufacturing an electrically heatable glow plug.

BACKGROUND INFORMATION

German Patent No. 19928037 describes an electrically heatable glow plugfor internal-combustion engines that includes a glow tube that is closedat its end and is corrosion-resistant, and that accommodates a fillingof a compressed, electrically nonconductive powder in which there isembedded an electrically conductive filament. The filament includes aheating coil. This heating coil is formed from an iron-chromium-aluminumalloy. In the area of the heating coil, the electrically conductivefilament is hardened on its surface. In this way, the filament canwithstand the mechanical stress during the compression process withoutdamage.

German Patent No. 19756988 describes an electrically heatable glow plugfor internal-combustion engines that has a glow element made of acorrosion-resistant metal jacket. In the glow element there is containeda compressed powder filling. An electrically conductive filament isembedded in the filling. In order to increase the life span of thefilament, a getter material is provided in the glow element for thebinding of the oxygen contained in the compressed powder filling. Thegetter material can be distributed in the compressed powder filling inthe form of electrically non-conductive particles. These particles canbe made of silicon or metal oxides of metals that oxidize in severaloxidation stages and that have a higher affinity to oxygen than does thefilament material; in the initial state, the getter material can containthe metal oxides in their first oxidation stage.

European Published Patent Application No. 0079385 describes a heatingelement in which a filament is situated in a sheath and is embedded inan electrically insulating powder. The powder has 0.1 to 10 weightpercent of an oxide, and in this way prevents the oxidation of themetallic portion of the filament.

SUMMARY OF THE INVENTION

In contrast, the electrically heatable glow plug and the method formanufacturing an electrically heatable glow plug have the advantage thatin the glow tube oxygen donors are provided, in order to form a layer ofaluminum oxide on the surface of the heating coil before or during theheating of the heating coil. In this way, in the case of a penetrationof air into the glow tube, the formation of nitrides in the edge layersof the heating coil, and thus a local increase of the electricalresistance and a premature failure of the heating coil, are prevented.

A further advantage is that an evaporation of aluminum from the alloycan largely be suppressed.

An economical realization of the supply of oxygen donors results whenthe heating coil in the glow tube is embedded in a first insulatingpowder, the first insulating powder including a material that acts as anoxygen donor.

It is particularly advantageous if the oxygen donor is formed as a metaloxide that can oxidize in several oxidation stages and that is presentin its highest oxidation stage. In this way, the oxygen release of themetal oxide is promoted considerably.

The same holds correspondingly if the oxidic ceramic powder includes ametal oxide that, under reducing conditions, can release oxygen throughdefect formation.

It is also advantageous if the oxygen donors are brought into the glowtube in the form of oxygen molecules under pressure. In this way,through the pressure the concentration of oxygen in the glow tube can beincreased, and through the oxygen molecules an oxidation can be realizedon the heating coil surface for the formation of aluminum oxide, withoutrequiring a heating of the heating coil by a heating current for thispurpose. In this way, the heating coil can be protected from nitridationby an oxide layer already before the first operation, i.e., before thefirst heating by a heating current.

A further advantage is that a control coil, connected to the heatingcoil, is embedded in a second insulating powder that is as free aspossible of oxygen donors and/or includes getter material for thebinding of oxygen. In this way, a material can be used for the controlcoil that does not form a protective oxide layer under the influence ofoxygen donors, as is the case for example for cobalt-iron alloys. Acorrosion of the control coil can thus be prevented, or at leastconsiderably delayed, through the use of the second insulating powderthat is as free as possible of oxygen donors.

With the use of getter material in the second insulating powder,disturbing oxygen molecules in the area of the control coil can bebound.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first exemplary embodiment of an electrically heatableglow plug according to the present invention.

FIG. 2 shows a second exemplary embodiment of an electrically heatableglow plug according to the present invention.

DETAILED DESCRIPTION

In FIG. 1, reference character 1 designates a glow plug, formed as asheathed-element glow plug, for an internal-combustion engine.Sheathed-element glow plug 1 includes a plug housing 40 having athreading 45 for screwing into a cylinder head of theinternal-combustion engine. Plug housing 40 further includes a hexagon50, via which the sheathed-element glow plug or plug housing 40 can bescrewed into or out of the cylinder head using a twisting tool, forexample a wrench for hexagon nuts. A glow tube 5 is pressed into plughousing 40, which is formed in the shape of a tube, and this glow tubeprotrudes from plug housing 40 at the side of the combustion chamber,i.e., at the end of plug housing 40 situated opposite hexagon 50. At theside of the combustion chamber, glow plug 5 is closed at its end. In anarea 20 at the combustion-chamber-side tip 55, formed in this way, ofglow plug 5, the cross-section of glow plug 5 can be reduced, as is thecase in this example. However, a reduction of this cross-section is notabsolutely necessary. Only area 20, having reduced cross-section, ofsheathed-element glow plug 1 protrudes into the combustion chamber. Inarea 20 having reduced cross-section, glow plug 5 has a heating coil 10that is welded to combustion-chamber-side tip 55 of glow tube 5.Adjoining heating coil 10 is a control coil 60, situated in the area ofglow tube 5, whose cross-section is not reduced. At the end of glow tube5 situated away from the combustion chamber, control coil 60 contacts aconnecting bolt 65 that can be connected with the positive pole of avehicle battery. In the direction towards the opening of plug housing 40situated away from the combustion chamber, glow tube 5 is sealed, stillinside plug housing 40, against environmental influences by a Viton ring70. A further sealing ring 75 seals connecting bolt 65, which protrudesfrom plug housing 40 away from the combustion chamber, against plughousing 40. An insulating disk 80, connected to sealing ring 75 awayfrom the combustion chamber, is used to electrically insulate connectingbolt 65 from plug housing 40, and thus electrically insulates connectingbolt 65 from plug housing 40, whose electrical potential is at vehicleground. A ring nut 85 presses insulating disk 80 onto plug housing 40,and presses sealing ring 75 into plug housing 40.

Glow tube 5 is of metallic construction, and, due to being pressed intoplug housing 40, its electrical potential is likewise at vehicle ground.Heating coil 10 is welded, with control coil 60, to a connection point90.

The function of Viton ring 70 is of considerable importance, because itis made of a soft, insulating material, and thus not only sealsconnecting bolt 65 in electrically insulating fashion against plughousing 40 at its end protruding into glow tube 5 for the contacting ofcontrol coil 60, but also prevents the penetration of air into glow tube5, which is open at its end away from the combustion chamber. Thissealing should be as reliable as possible.

Heating coil 10 is made for example of a ferritic steel having analuminum portion, for example of an iron-chromium-aluminum alloy. Thecontrol coil can for example be made of pure nickel or of a cobalt-ironalloy, having a portion of 6-18 weight percent cobalt, and has thefunction of a control resistance having a positive temperaturecoefficient.

In addition, in glow tube 5 an electrically insulating powder filling25, 30, which is compressed after the hammering of glow tube 5, isprovided, which ensures that heating coil 10 and control coil 60 in theinterior of glow tube 5 are housed and fixed in stationary fashion, aswell as being electrically insulated against glow tube 5, apart from tip55 of glow tube 5. As a powder filling, in general magnesium oxide isused. Moreover, the powder filling provides a thermal connection betweenglow tube 5 and heating coil 10, or control coil 60.

Given the presence of sufficient oxygen, the alloy of heating coil 10normally protects itself in a short time against further corrosionthrough the formation of a thin Al₂O₃ layer. However, this preconditionis not met in sheathed-element glow plug 1, due to an initial lack ofoxygen that is as a rule initially present. During the cyclical thermalloading of the sheathed-element glow plug in its use in the cylinderhead, air can penetrate into glow tube 5 despite sealing ring 75 andViton ring 70. This leads to a simultaneous reaction of the material ofheating coil 10 with oxygen and nitrogen. In contrast to oxygen, whichforms a protective aluminum oxide layer in the surface of heating coil10, nitrogen causes an interior nitridation, i.e., formation of aluminumnitride in the material of heating coil 10. The consequence is a localincrease of the electrical resistance of heating coil 10, resulting in ahigher voltage drop, and thus a greater heating at heating coil 10; thiscan cause a premature failure of heating coil 10.

For this reason, a material that acts as an oxygen donor is added to theinsulating powder filling, said material releasing oxygen at hightemperatures and thus promoting the formation of a protective aluminumoxide layer on heating coil 10. In this way, in the case of apenetration of air into glow tube 5, the formation of nitrides in theedge layers of heating coil 10 is prevented. The aluminum oxide layer ishere at least partially realized by a heating current already during thefirst heating of heating coil 10, in which temperatures of greater than1000 degrees Celsius are reached.

If the material of control coil 60 has no aluminum portion and also nosilicon portion, as in the example described here, then it does not forma protective oxide layer with the oxygen released by the oxygen donors,but rather corrodes. This should be prevented. For this reason, in thiscase the material of the insulating powder filling acting as an oxygendonor is added only in area 20 at tip 55 of glow tube 5, in whichheating coil 10 is located. The material acting as an oxygen donorshould thus be present only in the area of heating coil 10, and not inthe area of control coil 60. For this purpose, in the assembly ofsheathed-element glow plug 1, first glow tube 5 is filled with theinsulating powder having the material acting as an oxygen donor untilheating coil 10 is embedded therein as completely as possible, andcontrol coil 60 does not come into contact with the material acting asan oxygen donor even after a hammering of glow tube 5. The insulatingpowder filling enriched with the material acting as an oxygen donor isdesignated with reference character 25 in FIG. 1, and is referred to inthe following as the first insulating powder. The insulating powder withwhich glow tube 5 is subsequently filled, and in which control coil 60is embedded, should in this example contain no material acting as anoxygen donor, and should for example be formed from pure magnesiumoxide. In this way, the oxidation is supported only in the area ofheating coil 10, so that both a nitridation of heating coil 10 and acorrosion of control coil 60 can be prevented. The insulating powder,which is free of materials acting as oxygen donors, is designated inFIG. 1 with reference character 30, and represents a second insulatingpowder. Alternatively, or in addition, second insulating powder 30 caninclude a getter material for the binding of oxygen, such as for exampleSi, Ti, Al, or reduced metal oxides, such as for example FeO, Ti₂O₃.Given an electrically conductive getter material, such as for exampleSi, Ti, Al, second insulating powder 30 contains electrically insulatingmaterial, such as for example MgO, in a significantly greaterconcentration than the getter material.

The material acting as an oxygen donor can for example be formed as anoxidic ceramic powder. Here, the ceramic powder can be a metal oxide ofa metal that can oxidize in several oxidation stages. In order topromote the releasing of oxygen, in an initial state this metal oxidecan be present in its highest oxidation stage. Here, for example TiO₂can be used as an oxygen donor.

A further possibility is to use as an oxygen donor an oxidic ceramicpowder or metal oxide that releases oxygen under reducing conditions,such as those present in area 20 at tip 55 of glow tube 5 due to thealuminum portion of heating coil 10, so that a defect results in thecrystal grid of the relevant metal oxide due to missing oxygen atoms.ZrO₂ can for example be selected as such an oxygen donor.

A content of the material acting as an oxygen donor in first insulatingpowder 25 in a range from as low as approximately 0.1 weight percent upto approximately 20 weight percent has proven sufficient for theintroduction of the oxidation on heating coil 10 upon heating; theremaining portion of first insulating powder 25 can for example beformed by magnesium oxide.

FIG. 2 shows a second exemplary embodiment of a glow plug according tothe present invention, in which identical reference characters designatethe same elements as in FIG. 1. In contrast to the first specificembodiment according to FIG. 1, in the second specific embodimentaccording to FIG. 2 glow tube 5 does not have a control coil, but ratherhas an electronic control element 95 that is protected againstoxidation, which can for example include a temperature sensor and akeying, dependent on the determined temperature, of the current suppliedto heating coil 10, and which is not described here in more detail. Acontrol coil or a control element can also be omitted entirely.Moreover, instead of first insulating powder 25 and second insulatingpowder 30, a third insulating powder 15 is provided in the entire areaof glow tube 5, this third powder being made of an electricallyinsulating material, for example magnesium oxide, and being free ofoxygen donors. Heating coil 10 is connected with connecting bolt 65 viacontrol element 95; here control element 95 can also be situated as farfrom the combustion chamber as possible, so that it will not be heatedtoo strongly. It can now be provided that before the first operation ofsheathed-element glow plug 1, an opening 35 is bored into glow tube 5;here opening 35 should be situated outside area 20 at tip 55 of glowtube 5 having heating coil 10, because this area could be too sensitivefor a boring due to its reduced cross-section. If, however, there are nostability problems in area 20 at tip 55 of glow tube 5, it is alsoconceivable to make bored opening 35 there; i.e., directly in the areaof heating coil 10. Here, opening 35 is made only after heating coil 10and, if necessary, control element 95 have been brought into area 20 attip 55 of glow tube 5, and glow tube 5 has been filled with thirdinsulating powder 15. Only then is opening 35 bored into glow tube 5.Through opening 35, oxygen molecules are then brought into glow tube 5under a gas atmosphere with controlled partial pressure. This processcan for example last between approximately one hour and approximately 20hours; the limits of this time span can also be adjusted upward ordownwards. Subsequently, opening 35 formed by the boring is againclosed. The closing can for example take place through welding. Throughthe controlled partial pressure, the concentration of oxygen in glowtube 5 is increased. The higher the partial pressure is, the higher theconcentration of the oxygen in glow tube 5 becomes. Due to the highconcentration of oxygen, and above all due to the presence of pureoxygen molecules, an oxidation on the surface of heating coil 10 can beaccelerated, so that a passivation of heating coil 10 through theformation of a thin Al₂O₃ layer on the surface of heating coil 10 can berealized in a short time, already before or during the first operationof sheathed-element glow plug 1 in the internal-combustion engine, theAl₂O₃ layer here exercising a protective function and, in the case of apenetration of small quantities of air during the operation of thesheathed-element glow plug, preventing the formation of nitrides onheating coil 10. In this way, the life span of sheathed-element glowplug 1 can be increased. In this case, this takes place throughpre-oxidation of heating coil 10 before the first setting into operationof sheathed-element glow plug 1. Through corresponding predeterminationof the partial pressure for the bringing of oxygen into glow tube 5, andgiven corresponding predetermination of the time in which the oxygen isbrought into glow tube 5, a protective layer can be produced on heatingcoil 10 that is defined in its composition; in this example it is formedas an aluminum oxide layer.

If the oxygen brought into glow tube 5 in this way is also distributedoutside the area having heating coil 10 in glow tube 5, the use of acontrol coil susceptible to oxidation and corrosion is not recommendedin the second exemplary embodiment, and the use of a control elementthat is resistant to oxidation and to corrosion, as described forexample on the basis of control element 95, or the omission of a controlcoil or control element, is to be preferred.

1. An electrically heatable glow plug for an internal-combustion engine,comprising: an electrically conductive heating coil; and a glow tubeclosed at an end thereof, into which the electrically conductive heatingcoil is inserted, the electrically conductive heating coil being formedat least partially from a material including aluminum, wherein: anoxygen donor is provided in the glow tube in order to form an aluminumoxide layer on a surface of the electrically conductive heating coil oneof before and during a heating of the electrically conductive heatingcoil.
 2. The glow plug as recited in claim 1, wherein: the materialincludes an aluminum-iron-chromium alloy.
 3. The glow plug as recited inclaim 1, wherein: the electrically conductive heating coil is embeddedin a first insulating powder, and the first insulating powder includes amaterial that acts as the oxygen donor.
 4. The glow plug as recited inclaim 3, wherein: the material acting as the oxygen donor includes anoxidic ceramic powder.
 5. The glow plug as recited in claim 4, wherein:the oxidic ceramic powder includes a metal oxide of a metal that is ableto oxidize in several oxidation stages.
 6. The glow plug as recited inclaim 5, wherein: the metal oxide includes TiO2.
 7. The glow plug asrecited in claim 5, wherein: in an initial state the metal oxide ispresent in its highest oxidation stage.
 8. The glow plug as recited inclaim 4, wherein: the oxidic ceramic powder includes a metal oxide thatunder a reducing condition is able to release oxygen through defectformation.
 9. The glow plug as recited in claim 8, wherein: the metaloxide includes ZrO2.
 10. The glow plug as recited in claim 3, wherein: acontent of the material acting as the oxygen donor is in a range fromapproximately 0.1 weight percent to approximately 20 weight percent ofthe first insulating powder.
 11. The glow plug as recited in claim 1,wherein: the oxygen donor is introduced into the glow tube as oxygenmolecules under pressure.
 12. A method for manufacturing an electricallyheatable glow plug for an internal-combustion engine, comprising:forming an electrically conductive heating coil at least partially of amaterial including aluminum; inserting the electrically conductiveheating coil into a glow tube that is closed at an end thereof; andbefore operating the glow plug, introducing an oxygen donor into theglow tube in order to form an aluminum oxide layer on a surface of theelectrically conductive heating coil one of before and during a heatingof the electrically conductive heating coil.
 13. The method as recitedin claim 12, wherein: the material includes an aluminum-iron-chromiumalloy.
 14. The method as recited in claim 12, further comprising:inserting the electrically conductive heating coil into an area of a tipof the glow tube; and after the inserting into the tip of the glow tube,filling the glow tube with a first insulating powder that includes amaterial acting as an oxygen donor, so that the electrically conductiveheating coil is embedded as completely as possible in the firstinsulating powder.
 15. The method as recited in claim 14, furthercomprising: subsequent to the filling of the glow tube with the firstinsulating powder, filling the glow tube with a second insulating powderthat is at least one of: as free as possible of the oxygen donor, andincludes getter material for a binding of oxygen; and embedding in thesecond insulating powder a control coil.
 16. The method as recited inclaim 15, wherein: the control coil includes a cobalt-iron alloy andadjoins the electrically conductive heating coil.
 17. The method asrecited in claim 15, wherein: the second insulating powder is based onMgO.
 18. The method as recited in claim 12, further comprising:inserting the electrically conductive heating coil into an area of a tipof the glow tube; filling the glow tube with a first insulating powder;and after the inserting into the area of the tip of the glow tube andafter the filling of the glow tube, performing the following: boring anopening into the glow tube, introducing oxygen molecules under pressureinto the glow tube through the opening of the glow tube, and sealing theopening formed by the boring.
 19. The method as recited in claim 18,wherein: the sealing is performed by welding.
 20. The method as recitedin claim 18, wherein: the oxygen molecules are introduced into the glowtube for a predetermined time.
 21. The method as recited in claim 20,wherein: the predetermined time is between approximately one hour andapproximately 20 hours.